diff --git a/build/build.go b/build/build.go
index e5b2c2b8f..2a25bc8a4 100644
--- a/build/build.go
+++ b/build/build.go
@@ -160,7 +160,7 @@ func importWithSrcDir(bctx build.Context, path string, srcDir string, mode build
 	}
 	var pkg *build.Package
 	var err error
-	if mod != nil {
+	if mod != nil && mod.IsValid() && !mod.IsStd() {
 		if _, dir, typ := mod.Lookup(path); typ != fastmod.PkgTypeNil {
 			srcDir = dir
 			pkg, err = bctx.ImportDir(srcDir, mode)
@@ -490,15 +490,21 @@ type PackageData struct {
 type Session struct {
 	options  *Options
 	bctx     *build.Context
-	Mod      *fastmod.Package
+	mod      *fastmod.Package
 	Archives map[string]*compiler.Archive
 	Types    map[string]*types.Package
 	Watcher  *fsnotify.Watcher
 }
 
-func (s *Session) LoadMod(dir string) (err error) {
-	s.Mod, err = fastmod.LoadPackage(dir, s.bctx)
-	return
+func (s *Session) checkMod(pkg *PackageData) (err error) {
+	s.mod.Clear()
+	if pkg != nil && !pkg.Goroot {
+		err := s.mod.LoadModule(pkg.Dir)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
 }
 
 func NewSession(options *Options) *Session {
@@ -515,6 +521,7 @@ func NewSession(options *Options) *Session {
 		Archives: make(map[string]*compiler.Archive),
 	}
 	s.bctx = NewBuildContext(s.InstallSuffix(), s.options.BuildTags)
+	s.mod = fastmod.NewPackage(s.bctx)
 	s.Types = make(map[string]*types.Package)
 	if options.Watch {
 		if out, err := exec.Command("ulimit", "-n").Output(); err == nil {
@@ -556,7 +563,7 @@ func (s *Session) BuildDir(packagePath string, importPath string, pkgObj string)
 		return err
 	}
 	pkg.JSFiles = jsFiles
-	archive, err := s.BuildPackage(pkg)
+	archive, err := s.buildPackage(pkg)
 	if err != nil {
 		return err
 	}
@@ -579,6 +586,10 @@ func (s *Session) BuildFiles(filenames []string, pkgObj string, packagePath stri
 			Dir:        packagePath,
 		},
 	}
+	err := s.checkMod(pkg)
+	if err != nil {
+		return err
+	}
 
 	for _, file := range filenames {
 		if strings.HasSuffix(file, ".inc.js") {
@@ -588,7 +599,7 @@ func (s *Session) BuildFiles(filenames []string, pkgObj string, packagePath stri
 		pkg.GoFiles = append(pkg.GoFiles, file)
 	}
 
-	archive, err := s.BuildPackage(pkg)
+	archive, err := s.buildPackage(pkg)
 	if err != nil {
 		return err
 	}
@@ -609,7 +620,7 @@ func (s *Session) BuildImportPathWithPackage(path string, pkgData *PackageData)
 	if pkgData != nil {
 		srcDir = pkgData.Dir
 		if !pkgData.Goroot {
-			mod = s.Mod
+			mod = s.mod
 		}
 	}
 	pkg, err := importWithSrcDir(*s.bctx, path, srcDir, 0, s.InstallSuffix(), mod)
@@ -620,7 +631,7 @@ func (s *Session) BuildImportPathWithPackage(path string, pkgData *PackageData)
 		return nil, nil, err
 	}
 
-	archive, err := s.BuildPackage(pkg)
+	archive, err := s.buildPackage(pkg)
 	if err != nil {
 		return nil, nil, err
 	}
@@ -629,6 +640,14 @@ func (s *Session) BuildImportPathWithPackage(path string, pkgData *PackageData)
 }
 
 func (s *Session) BuildPackage(pkg *PackageData) (*compiler.Archive, error) {
+	err := s.checkMod(pkg)
+	if err != nil {
+		return nil, err
+	}
+	return s.buildPackage(pkg)
+}
+
+func (s *Session) buildPackage(pkg *PackageData) (*compiler.Archive, error) {
 	if archive, ok := s.Archives[pkg.ImportPath]; ok {
 		return archive, nil
 	}
diff --git a/go.mod b/go.mod
index 52eaefe21..ad5b6741e 100644
--- a/go.mod
+++ b/go.mod
@@ -13,7 +13,7 @@ require (
 	github.com/shurcooL/vfsgen v0.0.0-20181202132449-6a9ea43bcacd // indirect
 	github.com/spf13/cobra v0.0.5
 	github.com/spf13/pflag v1.0.5
-	github.com/visualfc/fastmod v1.2.0
+	github.com/visualfc/fastmod v1.3.3
 	golang.org/x/crypto v0.0.0-20200208060501-ecb85df21340
 	golang.org/x/sys v0.0.0-20200124204421-9fbb57f87de9
 	golang.org/x/tools v0.0.0-20200131143746-097c1f2eed26
diff --git a/go.sum b/go.sum
index 54775763a..50108c055 100644
--- a/go.sum
+++ b/go.sum
@@ -68,6 +68,10 @@ github.com/visualfc/fastmod v1.0.0 h1:3UcYFM+mzdk5AeKF7itO7D7rEv9Vkk0f2HOZi6KCMh
 github.com/visualfc/fastmod v1.0.0/go.mod h1:MjR7AyXKzcr5S9uEeJPI4xID37/q88a8FSBS/2H8AFc=
 github.com/visualfc/fastmod v1.2.0 h1:DwuyA4xcSzQ/dwAA75nO2J8YXr8h/ouLEzzhIKGfURA=
 github.com/visualfc/fastmod v1.2.0/go.mod h1:GGLvzTGsAMsVBMSLEnypACnwEj6syguJSV0qob6LveI=
+github.com/visualfc/fastmod v1.3.2 h1:epzPttZQB6zsll9g16s2s0TidibhYDRJOz9nin1jwWc=
+github.com/visualfc/fastmod v1.3.2/go.mod h1:IpOumy9gVomQ72T+yA7gayE3V1FCNlwr6if3mlqnasY=
+github.com/visualfc/fastmod v1.3.3 h1:NPT3Zog9LJT0IX8pSTYIYUiL/HUib5tVUrySZXMDLEU=
+github.com/visualfc/fastmod v1.3.3/go.mod h1:IpOumy9gVomQ72T+yA7gayE3V1FCNlwr6if3mlqnasY=
 github.com/xordataexchange/crypt v0.0.3-0.20170626215501-b2862e3d0a77/go.mod h1:aYKd//L2LvnjZzWKhF00oedf4jCCReLcmhLdhm1A27Q=
 go.starlark.net v0.0.0-20190702223751-32f345186213/go.mod h1:c1/X6cHgvdXj6pUlmWKMkuqRnW4K8x2vwt6JAaaircg=
 golang.org/x/arch v0.0.0-20171004143515-077ac972c2e4/go.mod h1:cYlCBUl1MsqxdiKgmc4uh7TxZfWSFLOGSRR090WDxt8=
diff --git a/tool.go b/tool.go
index 66107efcc..3efceeb2f 100644
--- a/tool.go
+++ b/tool.go
@@ -78,7 +78,7 @@ func main() {
 	compilerFlags.BoolVar(&options.Color, "color", terminal.IsTerminal(int(os.Stderr.Fd())) && os.Getenv("TERM") != "dumb", "colored output")
 	compilerFlags.StringVar(&tags, "tags", "", "a list of build tags to consider satisfied during the build")
 	compilerFlags.BoolVar(&options.MapToLocalDisk, "localmap", false, "use local paths for sourcemap")
-	compilerFlags.BoolVarP(&options.Rebuild, "force", "f", false, "force rebuilding of packages that are already up-to-date")
+	compilerFlags.BoolVarP(&options.Rebuild, "force", "a", false, "force rebuilding of packages that are already up-to-date")
 
 	flagWatch := pflag.NewFlagSet("", 0)
 	flagWatch.BoolVarP(&options.Watch, "watch", "w", false, "watch for changes to the source files")
@@ -136,10 +136,6 @@ func main() {
 					if err != nil {
 						return err
 					}
-					if !pkg.Goroot {
-						s.LoadMod(pkg.Dir)
-					}
-
 					archive, err := s.BuildPackage(pkg)
 					if err != nil {
 						return err
@@ -200,10 +196,6 @@ func main() {
 					if err != nil {
 						return err
 					}
-					if !pkg.Goroot {
-						s.LoadMod(pkg.Dir)
-					}
-
 					archive, err := s.BuildPackage(pkg)
 					if err != nil {
 						return err
@@ -338,9 +330,6 @@ func main() {
 					continue
 				}
 				s := gbuild.NewSession(options)
-				if !pkg.Goroot {
-					s.LoadMod(pkg.Dir)
-				}
 
 				tests := &testFuncs{BuildContext: s.BuildContext(), Package: pkg.Package}
 				collectTests := func(testPkg *gbuild.PackageData, testPkgName string, needVar *bool) error {
@@ -592,10 +581,6 @@ func (fs serveCommandFileSystem) Open(requestName string) (http.File, error) {
 		}
 		switch {
 		case isPkg:
-			if !pkg.Goroot {
-				s.LoadMod(pkg.Dir)
-			}
-
 			buf := new(bytes.Buffer)
 			browserErrors := new(bytes.Buffer)
 			err := func() error {